In accordance with the development of fine processing technology of semiconductors, the formation of micro field emission cathodes becomes possible. Spindt et al. proposed a cone type electron emission cathode, so that a micro field emission electron source has drawn attention (reference document 1: C. A. Spindt, J. Appl. Phys. Vol. 39, p. 3504 (1968)).
A structure and manufacturing method of a field emission cathode proposed by Spindt is shown as a first conventional example in FIGS. 11A to 11D.
Referring to FIG. 11A, on the surface of a conductive substrate 101, an insulating layer 102 and a metal film 103 that functions as a gate are formed in this order. Then, a small opening 104 penetrating the metal film 103 and the insulating layer 102 to expose the conductive substrate 101 is formed by a general photolithography process.
Referring to FIG. 11B, then, a sacrificial layer 105 made of alumina is vapor-deposited at a shallow angle with respect to the substrate 101 so as to cover the metal film 103. With this step, the opening diameter of a gate formed by the metal film 103 is reduced.
Referring to FIG. 11C, thereafter, a metal 106 such as molybdenum that becomes an emitter is vapor-deposited perpendicular to the substrate 101. Since the opening diameter of the gate is reduced when vapor deposition is carried out, a cone-shaped emitter (cathode) 107 is formed inside the small opening 104.
Referring to FIG. 11D, then the unnecessary sacrificial layer 105 and metal 106 are removed by a lift-off method by etching with respect to the sacrificial layer 105. This device is operated by emitting an electron into vacuum by applying an electric voltage to a metal film 103 from the tip of the emitter 107 and receiving the emitted electrode with an anode electrode (positive electrode) (not shown) additionally disposed opposite to the emitter 107.
Thereafter, there have been proposed methods for forming a cold cathode having the similar vertical structure with the tip of the emitter sharper by using a crystal anisotropy etching of silicon or dry etching and thermal oxidation (reference document 2: H. F. Gray et al., IEDM Tech Dig. P. 776 (1986); reference document 3: Betsui, “Digest of the conference of The Institute of Electronics, Information and Communication Engineers, Autumn, 1990, SC-8-2(1990)”). A structure and manufacturing method of a field emission cathode proposed by Betsui et al. is shown as a second conventional example in FIG. 12A to 12E.
Referring to FIG. 12A, on a silicon substrate 111, an oxide film 112 is formed. Referring to FIG. 12B, by using this oxide film 112, a disk-shaped etching mask 113 is formed by a photolithography process.
Referring to FIG. 12C, then, a tapered three-dimensional shaped portion 114 is formed under the etching mask 113 by carrying out a dry etching under the conditions where side etching is present. Furthermore, by carrying out thermal oxidation, the periphery of the three-dimensional shape portion 114 is changed into a thermal oxide film 115. Thereby, a cone-shaped portion 116 made of silicon is formed inside.
Referring to FIG. 12D, an insulating film 117 such as an oxide silicon film and a metal film 118 that functions as a gate electrode are vapor-deposited in the direction perpendicular to the surface of the substrate 111, thereby attaching the insulating film 117 and the metal film 118 onto the etching mask 113 and the thermal oxide film 115.
Referring to FIG. 12E, finally by soaking in hydrofluoric acid, a thermal oxide film 115 in the vicinity of a cone-shaped portion 116 is removed, and at the same time, the etching mask 113 to which the insulating film 117 and the metal film 118 are attached is removed, thereby forming an electron source having a structure similar to the structure of the above-mentioned Spindt type electron source.
This electron source is operated by applying an electric voltage to the metal film 118 that functions as a gate electrode so as to emit electron into vacuum from the tip 119 of the cone-shaped emitter 116, and receiving the emitted electrode with an anode electrode (positive electrode) (not shown) additionally disposed opposite to the emitter 116.
On the other hand, the present inventor group has proposed a tower-shaped electron source capable of operating at lower voltage (see, EP 637050A2). A manufacturing method of this towered-shaped electron source is shown as a third conventional example in FIG. 13A to 13H.
Referring to FIG. 13A, an oxide silicon film is formed on a (100) surface of a silicon crystal substrate 121 by a thermal oxidation method, and processed into a disk-shaped micro etching mask 122B having a diameter of 1 μm or less by photolithography.
Referring to FIG. 13B, then, by carrying out anisotropic dry etching with respect to the silicon substrate 121 using the micro etching mask 122B, a cylindrical body 124A made of silicon is formed under the micro etching mask 122B.
Referring to FIG. 13C, thereafter, by carrying out crystal anisotropic etching with respect to this cylindrical body 124A, a drum-shaped body 124B with a side face formed of a surface including (331) face and a top portion including a pair of opposite cylindrical bodies is formed.
Referring to FIG. 13D, then, a thin first thermal oxide film 125 is formed on the upper side of the drum-shaped body 124B and on the surfaces of the silicon substrate 121. Referring to FIG. 13E, thereafter, by carrying out an anisotropic dry etching with respect to a silicon substrate 121 by using a micro etching mask 122B, a column shaped body 124C is formed under the drum-shaped body 124B.
Referring to FIG. 13F, then, by a thermal oxidation method, on the surfaces of the drum-shaped column body 124C (FIG. 13E) and the silicon substrate 121, a second thermal oxide film 126 is formed. Thereby, inside the drum-shaped column 124C, a tower-shaped cathode 127 having a micro diameter and a steep tip portion is formed.
Referring to FIG. 13G, on the etching mask 122B and on the substrate 121 in the vicinity of the micro etching mask 122B, an insulating film 128 and a metal film 129 are sequentially deposited by vapor deposition.
Referring to FIG. 13H, furthermore, by carrying out wet etching with respect to a second thermal oxide film 126, the micro etching mask 122B and the insulating film 128 and metal film 129 formed on the micro etching mask 122B are removed. Thereby, the tower-shaped cathode 127 is exposed and at the same time, an extraction electrode 129A made of metal film having the same size as the inner diameter of the micro etching mask 122B is formed.
Since the electron source shown in the first to third conventional examples mentioned above has a micro diameter of a gate opening, a field emission current can be obtained with a relatively low voltage.
Furthermore, for the purpose of increasing the emission current, the present inventor group has proposed an electron source by forming a porous silicon film on a surface of the convex microstructure by an anodic oxidation method, thereby emitting electrons from micro protruding portions on the surface of the porous silicon film (JP 9 (1997)-270288A). A structure and manufacturing method of the electron source are shown as a fourth conventional example in FIG. 14A to 14E.
Referring to FIG. 14A, on the surface of a silicon substrate 131, a porous silicon layer 132 is formed by an anodic oxidation method. Referring to FIG. 14B, then, on the porous silicon layer 132, an oxide silicon film containing phosphorus is deposited by a CVD method, and furthermore, a disk-shaped etching mask 133 having a radius of about 1 μm is formed thereon by photolithography.
Referring to FIG. 14C, by dry-etching the porous silicon layer 132 and the silicon substrate 131 in the vicinity of the etching mask 133, a convex structure 136 is formed.
Referring to FIG. 14D, a silicon oxide film 134 and a metal electrode 135 are vapor-deposited by using an etching mask 133 as a mask for vapor deposition. Referring to FIG. 14E, finally, by soaking in hydrofluoric acid, the etching mask 133 is dissolved so as to remove the oxide silicon film 134 and the metal electrode 135 deposited on the etching mask 133. Thus, an electron source is completed.
In this case, by applying a voltage between the silicon substrate 131 and a metal electrode 135, an electric field is concentrated on the protruding tip on the surface of the porous silicon layer 132 formed by anodic oxidation and electrons are emitted. According to this method, on the surface of the porous silicon layer 132 formed inside the open portion of the metal electrode 135, substantially numerous protruding portions, which are formed by an anodic oxidation step, are formed, and electron beams are emitted from a large number of protruding portions. Thus, a field emission electron source with a large current density can be obtained.
However, in the electron sources described in the first to third conventional examples, in order to increase the current density, gate open portions corresponding to each emitter were required to be arranged in an array at high density. In these electron sources, since space between the emitters are separated from each other by an insulating layer, when the pitches between the opening portions are narrowed in order to increase the density of the emitter arrangement, the insulating layer as a separation wall becomes thin. Therefore, gate electrode may be peeled off.
When the film thickness of the insulating layer is made to be thin, the problem may be avoided. However, since the resistant voltage of the insulating layer is reduced, a voltage sufficient to extract electrodes cannot be applied. As a result, large current cannot be obtained.
On the other hand, in the fourth conventional example mentioned above, since a general semiconductor manufacturing line does not have an anodic oxidation step, this step is required to be added, thus increasing the cost. In addition, sufficient evaluation and analysis, etc. of the effect on the other steps is required. Furthermore, when the anodic oxidation step is actually added, many problems about mass production, for example, controllability of the anodic oxidation step and uniformity of the surface of the porous silicon layer, etc., have to be clarified.